This invention relates to a method for producing non-carbon, metal-based, anodes for use in cells for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte, and their use to produce aluminium.
The technology for the production of aluminium by the electrolysis of alumina, dissolved in molten cryolite, at temperatures around 950xc2x0 C. is more than one hundred years old.
This process, conceived almost simultaneously by Hall and Hxc3xa9roult, has not evolved as many other electrochemical processes.
The anodes are still made of carbonaceous material and must be replaced every few weeks. During electrolysis the oxygen which should evolve on the anode surface combines with the carbon to form polluting CO2 and small amounts of CO and fluorine-containing dangerous gases. The actual consumption of the anode is as much as 450 Kg/Ton of aluminium produced which is more than ⅓ higher than the theoretical amount of 333 Kg/Ton.
Using metal anodes in aluminium electrowinning cells would drastically improve the aluminium process by reducing pollution and the costs of aluminium production.
U.S. Pat. No. 4,614,569 (Duruz/Derivaz/Debely/Adorian) describes anodes for aluminium electrowinning coated with a protective coating of cerium oxyfluoride, formed in-situ in the cell or pre-applied, this coating being maintained by the addition of cerium to the molten cryolite electrolyte. This made it possible to have a protection of the surface only from the electrolyte attack and to a certain extent from the gaseous oxygen but not from the nascent monoatomic oxygen.
EP Patent application 0,306,100 (Nyguen/Lazouni/Doan) describes anodes composed of a chromium, nickel, cobalt and/or iron based substrate covered with an oxygen barrier layer and a ceramic coating of nickel, copper and/or manganese oxide which may be further covered with an in-situ formed protective cerium oxyfluoride layer.
Likewise, U.S. Pat. Nos. 5,069,771, 4,960,494 and 4,956,068 (all Nyguen/Lazouni/Doan) disclose aluminium production anodes with an oxidised copper-nickel surface on an alloy substrate with a protective oxygen barrier layer. However, full protection of the alloy substrate was difficult to achieve.
Metal or metal-based anodes are highly desirable in aluminium electrowinning cells instead of carbon-based anodes. As mentioned hereabove, many attempts were made to use metallic anodes for aluminium production, however they were never adopted by the aluminium industry.
A major object of the invention is to provide a method for manufacturing an anode for aluminium electrowinning which has no carbon so as to eliminate carbon-generated pollution and increase the anode life.
A further object of the invention is to provide a method for manufacturing an aluminium electrowinning anode with a surface having a high electrochemical activity for the oxidation of oxygen ions for the formation and evolution of bimolecular gaseous oxygen and a low solubility in the electrolyte.
Another object of the invention is to provide a method for manufacturing an anode for the electrowinning of aluminium which is covered with an electrochemically active layer with limited ionic conductivity for oxygen ions and at least a limited barrier to monoatomic oxygen.
Yet another object of the invention is to provide a method for manufacturing an anode for the electrowinning of aluminium which is made of readily available material(s).
The invention relates to a method of manufacturing an anode for use in a cell for the electrowinning of aluminium by the electrolysis of alumina dissolved in a fluoride-containing molten electrolyte, such as cryolite, at an operating temperature in the range of 700xc2x0 to 970xc2x0 C., preferably between 820xc2x0 and 870xc2x0 C. The anode comprises an iron-nickel alloy substrate.
A suitable electrolyte at a temperature of 820xc2x0 to 870xc2x0 C. may typically contain 23 to 26.5 weight % AlF3, 3 to 5 weight % Al2O3, 1 to 2 weight % LiF and 1 to 2 weight % MgF2.
According to the invention, the method comprises, before use in an electrolyte at an operating temperature in the above mentioned range, oxidising the iron-nickel alloy substrate in an oxygen-containing atmosphere at a temperature (hereinafter called the xe2x80x9coxidation temperaturexe2x80x9d) which is at least 50xc2x0 C. above the operating temperature of the electrolyte to form on the surface of the iron-nickel substrate a coherent and adherent iron oxide-containing outer layer having a limited ionic conductivity for oxygen ions and acting as a partial barrier to monoatomic oxygen. The outer layer is electrochemically active for the oxidation of oxygen ions and reduces also diffusion of oxygen into the iron-nickel alloy substrate when the anode is in use.
The iron oxide-containing outer layer may be a hematite-containing layer. At greater nickel concentration in the iron-nickel substrate, the iron oxide-containing outer layer may also contain nickel oxides, mainly nickel ferrite, in addition to iron oxide.
It has been observed that iron oxides and in particular hematite (Fe2O3) have a higher solubility than nickel and other metals in fluoride-containing molten electrolyte. However, in commercial production the contamination tolerance of the product aluminium by iron oxides is also much higher (up to 2000 ppm) than for other metal impurities.
Solubility is an intrinsic property of anode materials and cannot be changed otherwise than by modifying the electrolyte composition and/or the operating temperature of a cell.
Laboratory scale cell tests utilising a NiFe2O4/Cu cermet anode and operating under steady conditions were carried out to establish the concentration of iron in molten electrolyte and in the product aluminium under different operating conditions.
In the case of iron oxide, it has been found that lowering the temperature of the electrolyte decreases drastically the solubility of iron species. This effect can surprisingly be exploited to produce a major impact on cell operation by limiting the contamination of the product aluminium by iron.
Thus, it has been found that when the temperature of aluminium electrowinning cells is reduced below the temperature of conventional cells an anode provided with an outer layer of iron oxide which is obtained by the method of this invention can be made dimensionally stable by maintaining a concentration of iron species in the molten electrolyte sufficient to suppress the dissolution of the electrochemically active iron oxide anode surface obtained by the method of the invention but low enough not to exceed the commercially acceptable level of iron in the product aluminium.
As stated above, the method of the invention comprises oxidising, before use in an electrolyte of an aluminium electrowinning cell, the iron-nickel alloy substrate in an oxygen-containing atmosphere at an oxidation temperature which is at least 50xc2x0 C. above the operating temperature of the electrolyte.
However, the oxidation temperature can be 100xc2x0 C. or more above the cell operating temperature, in particular 150xc2x0 to 250xc2x0 C. above. Usually, the oxidation temperature is below 1250xc2x0 C. The oxidation temperature may for instance be from 950xc2x0 to 1150xc2x0 C., in particular from 1000xc2x0 to 1100xc2x0 C.
The oxidation period of the iron-nickel alloy substrate before use in an electrolyte may last 5 to 100 hours, in particular 20 to 75 hours.
The iron-nickel alloy may be oxidised in an oxygen-containing atmosphere having an oxygen-content between 10 to 100 weight %. For instance, the oxygen-containing atmosphere may be air.
The iron-nickel alloy substrate may comprise 30 to 95 weight % iron and 5 to 70 weight % nickel, in particular 40 to 80 weight % iron and 20 to 60 weight % nickel, for instance 50 to 70 weight % iron and 30 to 50 weight % nickel, i.e. with optionally up to 65 weight % of further constituents providing it is still capable of forming an iron oxide-based electrochemically active layer. Normally, the iron-nickel alloy comprises less than 40 weight %, in particular less than 20 weight % and often less than 10 weight %, of further constituents. Such constituents may be added to improve the mechanical and/or electrical properties of the anode substrate, and/or the adherence, the electrical conductivity and/or the electrochemical activity of the anode layer.
The iron-nickel alloy substrate may in particular comprise in addition to iron and nickel the following constituents in the given proportions: up to 15 weight % of chromium and/or additional alloying metals selected from titanium, copper, molybdenum, aluminium, hafnium, manganese, niobium, silicon, tantalum, tungsten, vanadium, yttrium and zirconium, in a total amount of up to 5 weight %. Furthermore, nickel present in the iron-nickel alloy substrate may be partly substituted with cobalt. The iron-nickel alloy substrate may contain up to 30 weight % of cobalt.
The invention also relates to a method of preparing an anode and operating it in an aluminium electrowinning cell which comprises at least one cathode and contains alumina dissolved in a molten electrolyte. The method comprises manufacturing an anode in an oxygen-containing atmosphere at a temperature which is at least 50xc2x0 C. above the operating temperature of the molten electrolyte as defined above, transferring the anode into the molten electrolyte contained in the aluminium electrowinning cell, and passing an ionic current from the anode to the cathode so that the alumina dissolved in the molten electrolyte is electrolysed to produce oxygen on the anode and aluminium on the cathode.
To avoid thermal shocks, the anode may be transferred into the molten electrolyte without cooling the anode below the temperature of the molten electrolyte.
During cell operation, the anode may be kept dimensionally stable in the molten electrolyte by maintaining a sufficient amount of dissolved alumina and iron species in the molten electrolyte to prevent dissolution of the iron oxide-containing outer layer.
As discussed above the cell may advantageously be operated at a sufficiently low temperature to limit the solubility of the iron oxide-containing outer layer, thereby limiting the contamination of the product aluminium by constituents of the iron oxide-containing outer layer.
The invention will be further described in the following Examples: